(a) Field of the Invention
The subject invention is in the field of seismic research and is used for exploration and detection of hydrocarbon deposits onshore and offshore, monitoring of producing oil and gas fields, and monitoring of storage of natural gas in natural subsurface reservoirs.
(b) Discussion of Prior Art
At present, substantially all work associated with exploration for hydrocarbon deposits involves, to a lesser or greater extent, seismic prospecting. Traditionally, seismic prospecting consists of recording reflected and refracted wave generated seismic vibrations propagated through the earth""s crust, with subsequent mathematical processing of obtained data Methods of seismic exploration know today make use of recorded seismic vibrations with frequencies ranging anywhere from 5-120 Hz. Significant strides have been made in these methods, as well as in mathematical methods for processing of the data. Predominately, vibrators or explosives have been used for the generation of such vibrations. Such procedures have negative ecological consequences for exploration areas. Furthermore, predictions of the presence of hydrocarbon accumulations based on known methods of seismic survey are not precise. Therefore, at least every other will drilled based on conclusions drawn from the traditional seismic exploration for hydrocarbon fields turn out to be erroneously placed. Besides the waste of resources used for drilling of such wells, there is also unfounded damage to the environment.
Heretofore, there has been known seismic prospecting which includes sourcing of seismic vibrations in exploration areas, recording of a data signal and subsequent mathematical processing of the recorded signal. The data signal is recorded with no less than two receivers of the seismic vibrations placed at some distance from each other. Me deficiency of this method of detection is its low accuracy.
A method of vibroseis seismic prospecting is also known and used for exploration of hydrocarbon fields, which includes generation of seismic vibrations with a seismic vibrator, recording of a seismic signal with three component receivers and mathematical processing of this signal. Seismic vibrations are generated within a 1-20 Hz range for not less than 3 minutes. Seismic vibrations are recorded for not less than 20 minutes before generating the seismic vibrations and then within 5 minutes before generating the seismic vibrations. It was proposed that the seismic background noise is to be recorded as a seismic signal. Presence of a hydrocarbon field was judged based on the area increase under the spectral curve during recording of seismic background after generation of seismic vibrations has stopped in comparison with recording of seismic background before generation of seismic vibrations started. The deficiency of this method is its low accuracy.
A primary object of the subject invention as described herein is development of a more precise method of exploration and detection of hydrocarbon deposits onshore and offshore.
Another object of this invention is development of a method for monitoring of producing oil and gas fields.
Still another object of the invention is the development of a method for monitoring the fill level of subsurface natural gas storage.
In one method of the invention at least one receiver if seismic vibrations capable of recording vibrations in the infrasonic range is placed over an area being surveyed. The spectral characteristics of the microseismic noise of the Earth recorded within a 2-5 Hz frequency range are used as the information signal, xe2x80x9cpassive variantxe2x80x9d. A hydrocarbon deposit is determined by the presence of a spectral anomaly on a spectrum of the passive information signal relative to a spectrum of an information signal from the area known not to contain hydrocarbons. In another method of the invention, seismic vibrations are generated over an area being surveyed using, as an example, a seismic vibrator. At least one receiver of seismic vibrations is placed over an expected hydrocarbon deposit. An xe2x80x9cactivexe2x80x9d information signal is recorded before and during the generation of seismic vibrations within a 2-5 Hz frequency range. A hydrocarbon deposit is determined by the presence of a spectral anomaly on a spectrum of the active information signal relative to the spectrum of the passive information signal at the same point. The invention provides for a direct method of detection of hydrocarbon deposits.
These and other objects of the present invention will become apparent to those familiar with geophysical oil and gas prospecting and acoustic research when reviewing the following detailed description, showing novel methods and detailed steps of the methods as herein described, and more particularly defined by the claims it being understood that changes in the embodiments to the herein disclosed invention are meant to be included as coming within the scope of the claims, except insofar as they may be precluded by the prior art.